I am trying to figure out what my compression ratio is on my new 383 motor.

 

Untouched they should be 54.4cc, but if they were milled it might be less.

 

This question was up about 2 weeks ago. You are about to get 15 different answers. Do a quick search and you will see what i'm talking about.

 

How can you "add" to the cc of a head? Like go from 54cc to say 58cc or higher>

 

Any grinding you do in the combustion chamber is going remove material which will add cc to the combustion chamber size. For example, if you deshroud the valve or do any work around the sparkplug it will increase the chamber size. Most ported heads (for NA motors) then get milled to compensate for this and typically get a lil extra milling to add some compression.

Where are you going next with that question?

-brent

 

Well that was me that posted under my little brother's call name. The reason I ask is because I just got my 383 shortblock built. Well my engine builder thought that my heads were 58cc when they are stock LT4 cc, which appears to be 54cc. With the 58cc it put my compression ratio at 11.8:1, but the engine builder said with the 54cc heads it puts it more around 12.2:1 I plan on driving this on the street some and it makes me nervous running 93 octane in it.

I was hoping I could possible get the heads done up to add CC to them to lower my compression ratio.

 

stock LT1= 54cc
stock LT4= 54.4cc

your engine builder should CC the heads himself to know exactly what he is dealing with

 

I agree with that.

Shortblock is already assembled? Cutting reliefs in the pistons is not very practical then.

You need to list your cam specs and determine your dynamic compression instead of being arbitrarly nervous about 12.2:1, it may turn out to be alright, but you don't know untill you calculate it. Read some posts on dynamic compression so you understand it, then ask for someone to double check your calcualtions.

are these LT4 heads stock or ported?

Do you ahve your cam picked out?

-brent

 

Yeah the heads are fully ported up. jordon musser did them. They have a valve job with 2.02/1.60 Ferrera valves. the intake manifold was also ported.
I believe the heads flowed like 286 at like .550 and 190 at .550

I also already have my cam selected. It is a custom ground Cam Motion solid roller camshaft. The specs of the cam are = 245.7/251.1, .654/.606 installed at a 111.8 Lobe seperation. The Duration is at .05

 

the different valves, change in valve seat height, any porting done to the chamber, and any milling the heads may have had will all affect the chamber size. you definilty need to have them CC'd

 

Is that on a 108 ICL? If so, you should be a hair under 9.0:1 on your DC, which is good to shoot for .

In any case, my static is right at 12.1:1, and 8.7:1 on the dynamic... I'm safe . And it was w/ smaller combustion chambers even. And only -5cc valve reliefs.

 

Shawn, I don't see the ICL on my cam sheet from Cam Motions. What is that the abbreviation for?

 

So you are saying with all the work done to the valve area, the CCs might be larger than 54.4??? Can a machine shop "measure" for the combustion chamber?

 

Intake center line.

Does it list the 'advance' that is ground in?

 

The larger valves reduced the CCs, but if jorden deshrouded the valves (99% chance he did) then that increased the chamber size. Bottom line is that you need to get them checked because these types of operations can greatly effect the chamber size.

Yes the machine shop should be able to if they aren't total hacks. Basically you flip the head over and fill it up with liquid and measure it with a syringe or some other measureing device.

You may want to ask jordan though. He may have measured them all to make sure they matched. You don't want one chamber smaller then all the rest and always detonating.

-brent